Known as multi-lamp EBs, electronic ballasts designed to operate different lamps, particularly lamps of different wattages, have been commercially available for some time. One problem in this context is that of ensuring soft switching of the inverter bridge circuit in the case of different loads.
In the following description it will be assumed that the inverter is equipped with a half bridge. As will be immediately obvious to the person skilled in the art, the following description is equally applicable to inverters with switches in a full-bridge configuration.
In a prior art Infineon controller for discharge lamps, switching during the conducting phase of the freewheeling diode via the second electronic switch is ensured as follows: the current in the lower branch of the bridge is measured using a half bridge shunt resistor. The undershooting of a negative threshold of the current is equated with the point in time at which the freewheeling diode of the lower switching element becomes conducting. This event triggers the closure of the lower half bridge switch and therefore determines the dead time of the control signals for the switches of the half bridge.
This form of control is problematic when the bridge circuit is operated at a frequency immediately above the phase shift, i.e. above the transition from inductive operation to capacitive operation at high loads. In this operating mode, the available current for recharging the trapezoidal capacitor may be very small. This poses the risk that the negative threshold of the current through the half bridge shunt resistor will not be reached. In this case the dead time control known from the prior art adjusts the maximum dead time, i.e. a maximum specifiable time duration. As a result, the switching operation of the lower half bridge switch is executed after the flow of current through the freewheeling diode has already been terminated. Since at this point in time the voltage across the lower half bridge switch is non-zero, the lower switch of the half bridge no longer operates in a soft-switched manner. This results in undesirable switching losses and overloading of the transistors involved. The latter results, among other things, in a reduction in the service life of electronic ballasts of this kind.
In order nevertheless to ensure reliable soft switching of the half bridge, the resonant circuit normally present can be designed with large resonant capacitances. However, this measure results in increased reactive currents and therefore undesirably large losses in the inverter.